In multiple industrial fields there arises the need for unwinding a weblike material from a reel for feeding it to a line for transformation or for processing. For example, in the field of paper converting, for the production of rolls of toilet paper, rolls of paper for domestic or industrial use, paper handkerchiefs or serviettes, one or more layers of paper known as “tissues” are unwound from one or more reels and fed to a line for transformation, which may comprise a rewinder and other machines arranged in series, to form rolls of a diameter equal to the diameter of the finished product and of large axial length. Said rolls or logs are then cut transversely and divided into a plurality of smaller rolls, which are sent on for packaging. Problems similar to those typical of the field of tissue paper may arise in other technological fields, for example in the processing of plastic film, adhesive tape, fabrics, non-wovens, cardboard and in general wherever there is the need to unwind a weblike material for feeding means or stations for processing.
For unwinding the reels, unwinding devices are used, which provide the unwinding motion to the reel by means of a central mechanism, i.e., one that transmits motion to the axis of the reel, or else by means of a peripheral mechanism, i.e., one that transmits motion by means of a member (typically a belt), which is in contact with the outer periphery of the reel. Unwinders of this type are described, for example, in EP-B-0822912 and in U.S. Pat. No. 5,730,389. Other unwinders for weblike material are described in GB-A-2294033, U.S. Pat. No. 3,202,376, WO-A-0056644.
The weblike material must be fed to the line for transformation at a tension that is substantially constant. This is rendered difficult by the fact that the reels from which the weblike material is unwound frequently present an irregular shape, for example an elliptical cross section or in any case a deformed and non-circular cross section.
Various systems have been studied for controlling the tension of the weblike material unwound from reels. According to a first approach, the weblike material is run over a dandy roll, i.e., one supported with a mobile axis of rotation, typically by means of a pair of oscillating arms that are hinged about an axis of oscillation parallel to the axis of rotation of the roll itself. When the tension of the weblike material tends to vary, for example on account of the irregularity of the cross section of the reel, the dandy roll shifts, so preventing a variation of tension in the weblike material. A position transducer, for example an angular position transducer, detects the displacement of the dandy roll and generates a feedback signal, which is used by a control unit for modifying the speed of feed of the weblike material, in particular by accelerating or decelerating the reel.
An unwinder device with a dandy roll for controlling the tension of the weblike material is described in U.S. Pat. No. 6,079,661.
This type of control is not satisfactory in that it is particularly unstable. In fact, a possible deformation of the reel causes an oscillation of the dandy roll with a frequency of the same order of magnitude as the frequency of the rotation of the reel. The feedback signal that derives therefrom causes cyclic accelerations and decelerations, with a similar frequency, of the speed of rotation of the reel. On account of the high inertia of the reels of weblike material, the correction made by this type of control is retarded with respect to the onset of the variation of tension, and hence is not effective.
According to a different approach, the weblike material is run over a guiding roller, to which there are associated load cells that detect the reaction on the axis of the roll, due to the load applied on the roll by the weblike material in tension. A possible irregularity of the cross section of the reel tends to have repercussions in the form of a periodic variation of the load applied on the guiding roller. This variation is detected by the load cells, which generate a feedback signal that is used for intervening on the speed of rotation of the reel with the purpose of making up for the oscillations in tension.
Also this solution is not satisfactory for the same reasons that render inadequate the solution that utilizes the dandy roll, and moreover induce greater jumps of tension in the weblike material, even though—as compared to the dandy-roll solution—it affords the advantage of simplifying the path of the weblike material.
Similar problems of control of the tension of a weblike material may arise also in situations different from the typical one of unwinding from a reel for feeding a station set downstream of the station comprising the unwinder. In general, problems of control of the tension of a weblike material may arise in all those situations in which the weblike material is fed from one to the other of two stations set in sequence (not necessarily adjacent to one another) along a feed path, when the one or the other of the stations may undergo oscillations in the speed of supply or of absorption of the weblike material itself.